Reddit reviews The Modern Recorder Player: Treble Recorder - Volume 1

I fully endorse this point. When I teach children, at the beginning of the lessons, we play the 'point and shoot' game. Balancing the descant (soprano) on the lower lip and right thumb only, they point the bell of the instrument to any part of the room and 'shoot'. With a bit of balancing practice, they can hit the ceiling. The point is that the need to develop a posture that allows for complete freedom of all the moving fingers. The use of a thumb rest on bigger instruments is often the result of bad posture and an underdeveloped sense of balance.

However, if the intention is only to play for fun, without the wish to develop a proper technique, then thumb rests are perfectly acceptable. It's partly the result of many people not realising that there is a recognised technique for recorder playing. Walter van Hauwe's books are good starts.